The present invention relates to a golf ball manufacturing method in which a specific thermoplastic polyurethane material is used as the cover material and injection-molded around a core. More specifically, the invention relates to a manufacturing method which is capable of obtaining a golf ball having a high rebound, excellent spin characteristics and excellent scuff resistance, and having also excellent flow properties of a cover material in injection-molding and manufacturability.
The use of polyurethane materials as golf ball cover materials has received attention in recent years. Polyurethane materials are broadly divided, based on the process used to make molded parts therefrom, into thermoset polyurethane materials and thermoplastic polyurethane materials. Molded parts made of thermoset polyurethane materials can be obtained by mixing under applied heat a urethane prepolymer having isocyanate end groups with a liquid starting material such as a polyol or a polyamine as the curing agent, then pouring the mixture directly into a mold and heating to effect a urethane curing reaction.
Numerous golf balls which use such a thermoset polyurethane material have been disclosed in the prior art (e.g., Patent Reference 1: U.S. Pat. No. 5,334,673; Patent Reference 2: U.S. Pat. No. 6,117,024; and Patent Reference 3: U.S. Pat. No. 6,190,268). Methods of molding thermoset polyurethane materials are described in, for example, Patent Reference 4: U.S. Pat. No. 5,006,297; Patent Reference 5: U.S. Pat. No. 5,733,428; Patent Reference 6: U.S. Pat. No. 5,888,437; Patent Reference 7: U.S. Pat. No. 5,897,884; and Patent Reference 8: U.S. Pat. No. 5,947,843.
Because moldings made of thermoset polyurethane materials lack plasticity when heated, the starting materials and molded parts cannot be recycled. Moreover, given the length of the heating and curing step and of the cooling step and given the great difficulty of controlling the molding time owing to the high reactivity of the starting materials under heating and their instability, the manufacturability of molded parts made of thermoset polyurethane materials, when used as specialty moldings such as golf ball covers (moldings which encase a core material), is regarded as inefficient.
By contrast, moldings made of thermoplastic polyurethane materials are not obtained by directly reacting the starting materials. Instead, a linear polyurethane material synthesized using starting materials and a production method which differ somewhat from those for the thermoset polyurethane materials described above is employed in the molding operation. Such a polyurethane material is thermoplastic, and thermoplasticized polyurethane materials have the quality of solidifying when cooled. Such polyurethane materials can thus be molded using an injection molding machine. Injection molding a thermoplastic polyurethane material requires a much shorter molding time than the molding time for a thermoset polyurethane material and moreover is suitable for precision molding, making it ideal as a process for molding golf ball covers. In addition, thermoplastic polyurethane materials are recyclable, and are friendly to the global environment. Golf balls made using thermoplastic polyurethane materials are disclosed in, for example, Patent Reference 9: U.S. Pat. No. 3,395,109; Patent Reference 10: U.S. Pat. No. 4,248,432; and Patent Reference 11: U.S. Pat. No. 4,442,282.
However, golf ball covers made with conventional thermoplastic polyurethane materials have been unable to satisfy all of the following properties in a golf ball: feel on impact, controllability, rebound, and scuff resistance when hit with an iron.
To address this need, Patent Reference 12: JP-A 9-271538 discloses a golf ball cover made using a thermoplastic polyurethane material having a high resilience. Yet, even this golf ball cover falls short in terms of its scuff resistance when hit with an iron.
Patent Reference 13: JP-A 11-178949 describes a golf ball cover which has a relatively good scuff resistance when hit with an iron and is composed primarily of the reaction product of a thermoplastic polyurethane material with an isocyanate compound. In this cover, an isocyanate compound such as a block diisocyanate or an isocyanate dimer is added as an additive to the thermoplastic polyurethane material. Addition is carried out during melt mixing under applied heat using an extruder or during injection molding, with the reaction being effected during molding.
However, in the molding of a cover according to JP-A 11-178949 above, the isocyanate compound is hard to handle because it loses its activity in the presence of moisture, thus making it difficult to obtain a stable reaction product. In the case of blocked isocyanates, which are highly resistant to moisture absorption, the blocking agent that dissociates under heating has a strong odor, making it unsuitable for use in molding covers. Moreover, when the isocyanate compound is in the form of a powder or a solution, it is difficult to control the amount of addition to the thermoplastic polyurethane material, making control of the golf ball cover properties a challenge. Furthermore, owing to melting point and melt viscosity differences between the thermoplastic polyurethane material and the isocyanate compound, slippage arises within the molding machine, which sometimes makes thorough kneading impossible to achieve. In this prior art, for the reasons given above, control of the effects of moisture within the cover material and of the amount of additive included therein has been inadequate, making it impossible to achieve golf ball covers which are fully satisfactory in terms of their scuff resistance-improving effects.
The preferred thermoplastic polyurethane material described in JP-A 11-178949 is based on an aliphatic isocyanate. However, this thermoplastic polyurethane material has a very high reactivity with isocyanate, making the reaction difficult to control. As a result, one problem is that gelation tends to arise before the mixture is used in injection molding, making it impossible to ensure sufficient plasticity. Another problem is that gelation sometimes occurs during the molding operation. Yet another problem is that the resin to be recycled sometimes gels, as a result of which it cannot be reclaimed. These problems have made it difficult to put the above technology to practical use.
Patent Reference 14: JP-B 58-2063 (U.S. Pat. No. 4,347,338) describes a method of manufacturing thermoset polyurethane molded parts which involves intimately mixing a compound having two or more isocyanate groups with a thermoplastic resin which does not react with isocyanate groups, blending the resulting mixture with a thermoplastic polyurethane material, then furnishing the blend to a molding machine and molding. However, the objects of this art are simply to improve the solvent resistance and the resistance to continuous and repeated friction; the published specification makes no mention of the use of this molding material as a golf ball cover material. Accordingly, there continues to exist a desire for a golf ball cover material which can satisfy a number of properties required of golf balls; i.e., rebound, distance, spin characteristics, controllability, feel on impact, scuff resistance, cut resistance, and discoloration resistance.
Patent Reference 15: JP-A 2002-336378 discloses a golf ball which uses a cover material composed of a thermoplastic polyurethane material and an isocyanate mixture. The cover material is a thermoplastic polyurethane material which is recyclable, has a high rebound, and has an excellent scuff resistance. Such a cover material has both a good thermoplastic polyurethane manufacturability and exhibits physical properties comparable with those of thermoset polyurethanes. At the same time, the plasticizing effect of the isocyanate compound enhances the flow properties of the thermoplastic polyurethane material, thus enabling the manufacturability to be improved as well. However, this excellent art also has a number of drawbacks, including the generation of scorched material when the isocyanate mixture is charged directly into the molding machine, and variability in the mixing proportions due to the use of a dry blending process, which results in a poor uniformity and leads to poor molding stability. In addition, because the relative proportions within the isocyanate mixture of the isocyanate compound and the thermoplastic resin which substantially does not react with isocyanate are already decided, it has been difficult to freely select the amounts and types of the isocyanate compound and the thermoplastic resin that one wishes to add.
In addition, Patent Reference 16: JP-A 2002-336380 describes a golf ball wherein a material obtained by blending a thermoplastic polyurethane material that contains as the polymeric polyol a polyether polyol having an average molecular weight of at least 1500 and that has a rebound resilience of at least 40% with a specific isocyanate mixture is used as the cover material. However, this cover material has the same drawbacks as those associated with the art disclosed in aforementioned Patent Reference 15; namely, the generation of scorched material when the cover material is charged into the molding machine, poor molding stability, and limitations on selecting the amounts and types of isocyanate compound to be added.